The Earth, the Sun and the Moon

Days, months and years

• A day, a month and a year all come from motion in space.
• The Earth spins, the Moon 月球 circles us, and we circle the Sun.
• Let's see what each motion gives us — and how to find an orbital speed 轨道速率.

The spinning, orbiting Earth

• The Earth rotates 自转 on its axis once in about $24$ hours — giving day and night.
• It orbits 公转 the Sun once in about $365$ days — one year.
• Its axis is tilted, so each half of the Earth leans toward the Sun for part of the year — giving the seasons 季节.

The full Moon, Earth's natural satellite

The Moon

• The Moon orbits the Earth once in about one month.
• As it goes round, we see different amounts of its lit side — the phases 月相 (new, half, full).
• From shortest to longest: day (Earth's spin) < month (Moon's orbit) < year (Earth's orbit).

The Moon's lit half always faces the Sun, so as it orbits we see different amounts of it — new, half and full

Saturn and its rings, photographed by the Cassini spacecraft

Orbital speed

• One full orbit covers the circumference $2\pi r$, where $r$ is the orbit radius.
• The orbital speed is that distance divided by the period 周期 $T$ (the time for one orbit):

• The same equation works for planets, moons and satellites.

Mars, the red planet, one of the rocky inner planets

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